The surface chemistry of heterogeneous catalysis - Wiley Online Library

THE CHEMICAL RECORD

The Surface Chemistry of Heterogeneous Catalysis: Mechanisms, Selectivity, and Active Sites FRANCISCO ZAERA Department of Chemistry, University of California, Riverside, California 92521

Received 25 February 2005; Accepted 10 March 2005

ABSTRACT: The role of chemical kinetics in defining the requirements for the active sites of heterogeneous catalysts is discussed. A personal view is presented, with specific examples from our laboratory to illustrate the role of the chemical composition, structure, and electronic properties of specific surface sites in determining reaction activity and selectivity. Manipulation of catalytic behavior via the addition of chemical modifiers and by tuning of the reaction conditions is also introduced. © 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 5: 133–144; 2005: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20040 Key words: catalysis; surface chemistry; selectivity; hydrocarbon conversion; transition metals; oxides; chiral modification

Introduction The use of catalysis in chemistry has been ubiquitous for more than a century but still remains at the center of most industrial processes, including the manufacturing of commodity, fine, specialty, petro-, and agrochemicals, and the production of pharmaceuticals, cosmetics, foods, and polymers.1,2 Catalysis is also central to the generation of clean energy and to the protection of the environment, both by helping in the abatement of environmental pollutants and by providing cleaner chemical synthetic procedures. At present, catalysts are used in over 80% of all chemical industrial processes, create annual global sales of about 1500 billion dollars, and contribute directly or indirectly to approximately 35% of the world’s GDP.3,4 The term catalyst was coined by Berzelius in 1835 to refer to a substance that increases the rate of chemical reactions without being itself consumed.2,5–7 Berzelius’ definition highlights the fact that, at the beginning at least, the focus in catalysis was on modifying total reaction rates. As industrial

manufacturing has become more sophisticated, however, additional demands have been placed on catalytic processes to make them highly selective, to save on reactants, minimize separation processes, and avoid the need for expensive clean ups and disposals of unwanted byproducts. Selectivity, like activity, is defined by kinetics, but the parameters that control selectivity are typically more sensitive and difficult to adjust, and not necessarily the same, than those that define the total rate.8,9 Fortunately, there are many variables at our disposal to improve catalytic performance, in particular the properties of the catalysts used. Most catalytic processes are heterogeneous in nature, typically involving a solid catalyst (often an active phase finely dispersed on a high-surface-area support) and gas- or liquid-phase reactants. Compared to homogeneous catalysts, heterogeneous

Correspondence to: Francisco Zaera; e-mail: [email protected]

The Chemical Record, Vol. 5, 133–144 (2005) © 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

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catalysts offer several inherent advantages in terms of stability, low cost, and low toxicity. They are also easier to prepare, handle, separate from the reaction mixture, recover, and re-use. More to the point of this review, the use of solids in catalysis affords the buildup of complex atomic ensembles especially tuned for specific reactions. This flexibility in structural design can be utilized to develop processes with improved selectivity. Traditional preparation methods have offered limited control on the size and shape of catalytic surfaces, but recent advances in nanotechnology are starting to provide a variety of new tools for the manufacturing of novel materials with unique and welldefined structures and properties designed at the nanoscale. In order to properly direct those new technologies to advance heterogeneous catalysis, a clear vision is needed of the criteria for improving catalytic performance. For this, the first step must be the identification of the nature of the ensemble of surface atoms that constitute the best active site. Remembering that the foundations of catalysis rest on chemical kinetics, it becomes clear that the answer to that query goes back to the need to understand the mechanism of the reactions involved at a molecular level. In our laboratory we have worked for many years on trying to identify the requirements imposed on active sites by the mechanisms of catalytic reactions. Our approach has relied on the use of modern surface-sensitive analytical techniques and model catalytic systems. Below, a few recent examples from those studies are provided to illustrate the value of mechanistic information to the design of better catalysts.

Islanding of Adsorbates Soon after it became possible to study the mechanisms of catalytic reactions at a molecular level by using model surfaces, it was learned that even simple surface steps often display significant deviations from classical Wigner–Polanyi kinetic behavior.10–12 In particular, it was established that even on uniform and defect-free surfaces kinetic complications may arise from the fact that average surface concentrations do not necessarily reflect the local distribution of species around the ensemble where the catalytic reactions occur. Adsorbates often interact strongly with one another,13–15 and may end up distributed in a non-homogeneous manner across the surface by, for instance, associating into local two-dimensional islands. This behavior is of particular relevance to the kinetics of catalytic processes, because neighboring adsorbates modify the energetics of surface reaction in ways not explained by macroscopic kinetic models.12,16 An example of the importance of islanding in catalysis is provided by results from our studies on the oxidation of carbon monoxide on Pt(111) single-crystal surfaces.17,18 There, at least two kinetically distinct types of oxygen atoms were identified, in spite of the fact that all Oads species sit in identical sites at the start of the reaction. It was suggested that nearby oxygen atoms weaken the adsorption energy of adsorbed carbon monoxide molecules, thus enhancing their reactivity. In fact, it was determined directly that when oxygen is adsorbed in islands on the Pt(111) surface, the rate of CO oxidation is up to four times faster than when the same

Francisco Zaera was born in 1958, and received a Licenciate degree in Chemistry from Simón Bolívar University in Caracas, Venezuela in 1979. After working there as a lecturer for another year, he went on to get his Ph.D. degree in Chemistry from the University of California, Berkeley in 1984. He then took a position as Assistant Chemist at the Brookhaven National Laboratory National Synchrotron Light Source (in a joint appointment with Exxon Research Laboratories), and moved to his present position as Professor of Chemistry at the University of California, Riverside in 1986. Prof. Zaera has been awarded the 1994 and 1995 Union Carbide Innovation Recognition Program Award, the 2001 American Chemical Society George A. Olah Award in Hydrocarbon or Petroleum Chemistry and the 2003 Paul H. Emmett Award of the North American Catalysis Society, and is a Fellow of the American Association for the Advancement of Science. He has held several editorial offices, including his present post as American Editor of The Journal of Molecular Catalysis A: Chemical, and has organized more than ten conferences and symposia. He has also held several professional offices, including those of Treasurer (1997, 1998) and Vice-Chair (2005) of the Colloids and Surface Chemistry Division of the American Chemical Society, and Treasurer-Secretary (1990, 1991) and President (1992) of the California Catalysis Society. Prof. Zaera has close to 200 publications in scientific journals, and has presented over 120 invited talks. His research interests are in the areas of surface and materials chemistry and heterogeneous catalysis, with particular emphasis on surface reaction kinetics and in-situ spectroscopic characterization of surface species.

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© 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

Surface Chemistry of Heterogeneous Catalysis

Adsorption Geometries Reaction selectivity can also be determined by the geometry adopted by the adsorbate(s) on the surface of the catalyst. Intuitively, it can be argued that the reactivity of a specific bond in


3.0

14

N+15N/Rh(111) TPD Yields Evidence for Islanding

2.5

Y14N14N/Y14N14NStat

number of oxygen surface atoms are randomly distributed on the metal surface.17 Similar but more complex behavior was observed during the reduction of nitrogen monoxide on rhodium surfaces.19–21 Here, again, a single surface species follows several parallel kinetic trends. At one end, a critical coverage of atomic nitrogen builds up on the surface during an initial induction period before the start of the production of N2, and recombines and desorbs as N2 only upon thermal activation at the end of steady-state beam experiments. At the other extreme, a small amount of surface intermediates capable of producing molecular nitrogen forms during the catalytic process but disappears rapidly after the removal of the gas-phase reactants. Kinetic measurements showed that it is the concentration of the second species that correlates with the kinetics of NO reduction.20 However, additional isotope-switching experiments indicated that the two species are not intrinsically in different adsorption sites, but rather kinetically distinct because their local adsorption energies are affected by the inhomogeneous distribution of atomic nitrogen on the surface.22 Figure 1 displays evidence for this in the form of the non-statistical distribution of isotopes seen in the molecular nitrogen that desorbs upon heating surfaces prepared by catalytic conversion of NO + CO mixtures where the isotope labeling is switched midway from 14NO to 15NO. Specifically, the data highlight the fact that the yields of the 14N14N isotopomer are significantly higher than those expected on statistical grounds.23 Monte Carlo simulations were performed to explain the observed isotopic distributions in terms of the formation of “onion-like” two-dimensional islands with a 14N core surrounded by a 15N outer shel.24 Complementary experiments were carried out to determine the rates of production of all the possible molecular nitrogen isotopomers directly during the NO + CO catalytic conversion (rather than by thermal activation of the surface afterwards, as in the experiments used to obtained the data in Figure 1).25,26 Surprisingly, it was found that no 14N14N is ever produced after switching from 14NO to 15NO in the reaction mixture, indicating that a N¶NO intermediate must form before N2 production, most likely at the periphery of large atomic nitrogen islands.27 This suggests that the reported dependence of NO conversion on catalyst structure28 may be due to the need for surfaces with large (111) terraces to accommodate the nitrogen islands that support the formation of the N¶NO species.

Experimental Data

2.0 MC Simulations (Islanding) F = 9 atoms No Difussion

1.5

F = 9 atoms Difussion

F = 5 atoms Difussion

1.0 0.2

0.3

0.4 15

0.5

0.6

0.7

0.8

N Fraction

Fig. 1. 14N14N temperature-programmed desorption (TPD) yields (normalized to the statistical expectation) as a function of isotopic composition for the atomic nitrogen layers that grow on Rh(111) surfaces during the steady-state reduction of NO by CO.24 The symbols correspond to the experimental data obtained after sequential molecular beam kinetic runs using 14NO + CO and 15 NO + CO mixtures. A clear departure from statistical predictions is seen in the experimental data, indicative of a non-homogeneous distribution of the adsorbates on the surface. The solid lines correspond to results from Monte Carlo simulations assuming different island sizes with and without atomic diffusion to show that island formation can account for the observed experimental deviations. This suggests that flat terraces with a minimum width, sufficient to sustain these nitrogen islands, may be required for the optimal performance of rhodium catalyst in nitrogen oxide reduction reactions.

a chemisorbed species may depend on its degree of interaction with the surface, and that such interaction is likely to be determined by the degree of proximity between the two.15 For instance, it has been established that the initial adsorption of carbon monoxide on most transition metals is with its C¶O bond perpendicular to the surface plane.29 On the other hand, some degree of molecular tilting is likely to be required for the dissociation of CO in processes such as methanation and Fischer–Tropsch. Interestingly, tilted geometries have been reported on Cr(110),30 Fe(100),31 and Mo(100)32,33 surfaces. Those tilted species display low C¶O stretching frequencies and unusually long C¶O bonds, and a particular propensity toward C¶O bond scission. A nice correlation appears to exist in these systems between adsorption geometry and catalytic activity.

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Surface Defects More direct control on selectivity may be exerted by shaping the surface of the catalyst to optimize the transition state of the desired reaction. Unfortunately, even with the latest advances in nanotechnology, this conceptually simple idea is still quite difficult to implement in practice. Nevertheless, some advances have been made in terms of understanding the demands placed on the active site by the catalytic reaction in terms of its geometry and/or electronic properties. For one, surface-science studies have provided ample evidence for the high activity displayed by steps and other surface defects toward many surface reactions.49,50 Moreover, kinetic measurements of catalytic processes on model single crystals with specific surface planes exposed have provided a way to directly

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94

Cinchona/Pt Modification Enantioselectivity vs. Adsorption Geometry

Excess Enantioselectivity / %

Adsorption geometries are often affected by the coverage of the adsorbates on the surface.8,34,35 Such changes have been well documented for aromatic compounds, where strong p interactions between the organic ring and the metal, which dominate at low coverages and favor a flat adsorption geometry, are overtaken by p–p intermolecular attractions at higher coverages, forcing the aromatic rings to stack in a tilted geometry.36,37 We and others have also provided clear evidence for changing degrees of tilting with increasing coverages in aliphatic adsorbed hydrocarbons such as halo hydrocarbons,34 alkyls,35 and alkoxides.39 Again, these geometrical changes are accompanied with variations in reactivity: the more upright adsorption configurations seen at coverages close to monolayer saturation are typically less prone to dehydrogenation, and favor hydrogenation and/or coupling reactions instead.40 A particularly exciting possibility has been identified for the control of catalytic behavior via the manipulation of adsorption geometries in enantioselective processes where chirality is bestowed to regular catalysts by the addition of chiral modifiers.41 This has been clearly proven for the case of the hydrogenation of a-keto esters on platinum surfaces modified with cinchona alkaloids.42,43 Figure 2 correlates the geometry of cinchonidine adsorbed from a solution onto a platinum surface, as determined by in-situ infrared measurements,44–47 with enantioselectivity data from catalytic kinetic runs on a similar system.48 It is seen there that the performance of the catalyst, as determined by the enantiomeric excess obtained during the hydrogenation of ethyl pyruvate, is optimized by a flat-lying adsorption geometry of the aromatic ring of the cinchona modifier. It was also determined that that geometry depends strongly on the concentration of the modifier in solution, peaking at intermediate values.44–46 This provides us with a handle on how to control selectivity by tuning the conditions of the catalytic process.

92

90

Increasing cinchona concentration in solution

88 Orientation by IR: Adsorption of cinchonidine in CCl4 solution

86

Enantioselectivity for hydrogenation of ethyl pyruvate in acetic acid using 10,11-dihydrocinchonidine

84 0.01 QFlat cinchona / ML

0.1

Fig. 2. Correlation between adsorption geometry, as determined by in-situ infrared spectroscopy,44 and catalytic enantioselectivity48 in the case of the hydrogenation of ethyl pyruvate by cinchonidine-modified platinum. Optimal catalytic performance is obtained with a flat-lying geometry for the aromatic ring of the adsorbed cinchona, which is attained at intermediate solution concentrations, between 5 and 20% of saturation. This is a clear example of how catalytic performance can be affected by adsorption geometry, and of how that geometry can in turn be controlled by tuning the conditions of the reaction.

correlate the structure of the active site with its activity toward a particular reaction.51–54 For instance, the (111) plane of iron surfaces has been shown to be several orders of magnitude more active than the (110) orientation for both nitrogen activation55 and ammonia production.56 This behavior has been explained by the particularly high catalytic activity of iron atoms coordinated to seven iron neighbors.56,57 We have over the years carried out extensive research on the parameters that control selectivity in hydrocarbon conversion reactions. Early data were reported on the effect of surface structure on selectivity for alkane reforming over platinum catalysts.58,59 More recently, we have advanced the notion that the observed selectivities in those reactions may be defined by early dehydrogenation steps from key adsorbed intermediates.9,60–63 It is widely accepted that the overall rate of most alkane conversion processes is controlled by the activation of one of its C¶H bond.51,64 However, it is our contention that selectivity is likely to depend on the regioselectivity of the next dehydro-



CH3I/Metal Methane TPD

Dependence on Surface Structure Partial Pressure / arb. units

(a) Copper

468 K

(b) Nickel 238 K

470 K

Exposure = 1.0 L Tads = 300 K b = 2.5 K/s

Exposure = 5.0 L Tads = 100 K b = 10 K/s

237 K 460 K 255 K

275 K

300

350

400

450

500

Cu(110)

Ni(110)

Cu(100)

Ni(100)

Cu(111)

Ni(111)

550 600100 150 Temperature / K

200

250

300

350

400

Fig. 3. Methane temperature-programmed desorption (TPD) traces from iodomethane adsorbed on the (111), (100) and (110) planes of copper (a, left)98 and nickel (b, right)88,99,100 single-crystal surfaces. Methane formation, a reaction limited by a-hydride elimination from surface methyl groups, occurs at approximately the same temperature on all three surface planes of each metal, but at much higher temperatures on copper than on nickel. This points to the fact that alkyl dehydrogenation is affected to a much larger extent by the electronic properties of the surface than by its structure.

genation step from the resulting chemisorbed alkyl intermediates. Our experiments have clearly indicated a preference for dehydrogenation at the b position,65–72 a step that explicitly accounts for the fast production of olefins during reforming,73,74 and together with C ¨C insertions into metal–hydrogen bonds and alkyl–hydrogen reductive eliminations also explains H¶D exchange and double bond migration.40,71,74–89 Our most recent experiments with C4 alkyls, alkenes, and allylic intermediates have provided corroborating evidence for the role of all those steps in the interconversion between 1and 2-butene and between cis- and trans-2-butenes.90 It appears that the extent of cis–trans isomerization may be affected by the structure of the catalyst, and that close-packed planes may favor cis isomers while more open surfaces may facilitate the formation of the trans configuration.91,92 In spite of the ease with which alkyl surface moieties undergo b-hydride elimination, dehydrogenation at other positions in the hydrocarbon chain must also be possible,93,94 and are needed to account for the more demanding reforming processes. Past studies on supported catalysts suggest that while hydrogen removal at the g carbon may be responsible for desirable isomerization and cyclization reactions,95,96 dehydrogenation at the a position is likely to lead to the production of undesirable hydrogenolysis products instead.97 Both the


absolute reactivity and the selectivity among the hydride elimination steps from adsorbed hydrocarbon moieties should in principle depend on the structure of the surface, and that effect should be the most pronounced in cases where large surface atomic ensembles are required. Unfortunately, due to significant limitations with the experimental surface-sensitive techniques available to date, the understanding of the coordination details of hydrocarbon intermediates to surface metal atoms has not advanced much. In alkyl conversion, coordination has been proposed to occur on hollow multi-coordinated sites,72 but no hard evidence exists for this claim, and analogy with organometallic complexes suggests a single terminal metal–carbon bond instead.63,70 Regardless, in the few instances where the reactivity of alkyl species has been probed as a function of the structure of the surface, only subtle effects have been observed. Figure 3 shows an example for the case of a-hydride elimination from methyl surface moieties, which is followed by the rapid desorption of methane. It can be seen there that the temperature of the CH4 desorption peaks on both copper98 and nickel81,99,100 only changes slightly across the (111), (100), and (110) series of surface planes. Nevertheless, this type of structure sensitivity work has only been performed in a few cases,81,101 and the data are too scarce still to draw meaningful conclusions.

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35

Hydride Elimination Kinetics

30

Activation Energy vs. Metal d Character

Activation Energy / kcal·mol

-1

25 20 a-H elimination from CH3,ads

15 10 b-H elimination from C2H5,ads

5 15

DE a(a-H – b-H)

10

Cu

5

Pd

0

Ni

Pt

Rh

-5 35

40 45 % d Character

50

Fig. 4. Comparison of activation energies for a- and b-hydride elimination reactions across a family of metal surfaces. Individual activation barriers, calculated from temperature-programmed desorption data for adsorbed methyl and ethyl surface species, respectively,70,72,170 are plotted versus the %d character of the metal.171 In general, higher activities are seen for both reactions with early transition metals, but a clear preference for b- versus a-hydride elimination is also observed with decreasing %d character. These data provide an example of the effect that electronic properties exert on catalytic selectivity.

affected to the same extent by the nature of the surface. Consequently, the selectivity in alkyl dehydrogenation reactions also depends strongly on the metal used as catalyst. In fact, the data in Figure 4 suggest a particular relative preference for bhydride elimination on metals with low %d character. More elaborate experiments on Pt(111), based on both H¶D exchange studies in methyl groups78,80,81,99,109,110 and a direct comparison of a- vs. b-H elimination from ethyl surface moieties,82,85 are consistent with those findings. Additional steps are available and compete favorably with a- and b-H eliminations during alkyl conversion on certain metal surfaces. For instance, coinage metals, silver in particular, often favor alkyl-coupling reactions.111 Also, while only a-hydride elimination followed by hydrogenolysis (C¶C bond-scission) is seen in the case of neo-pentyl moieties on Ni(100),88,94 competition with g-hydride elimination from the same species is possible on platinum surfaces.112–115 This difference explains the unique ability of platinum to catalyze reforming processes and the preference for nickel to promote cracking instead.88 Again, the main point here is that what matters in terms of defining selectivity in catalysis is the relative rates among the different available paths on a given surface, not their absolute values. All a, b, and g hydride elimination steps may follow similar qualitative trends across the periodic table, but their relative rates also change significantly, and it is these latter changes that explain the different performance of the various metals in hydrocarbon reforming in terms of selectivity.

Chemical Composition of Active Sites Electronic Effects What is clear from the data in Figure 3 is that the activity of copper and nickel surfaces toward a-hydride elimination are quite different. Note in particular that much higher temperatures are needed for methyl dehydrogenation on the former metal. In general, the activity and selectivity of most hydrocarbon dehydrogenation reactions depend strongly on the electronic properties of the catalytic surface. To further illustrate this point, Figure 4 contrasts the activation energies estimated from reported temperature-programmed desorption data for a-hydride elimination from methyl groups versus b-hydride elimination from ethyl surface species on a number of late transition metals.66,93,99,102–108 This figure makes clear that on most metals hydride elimination from the a carbon is more demanding than from the b position. Also highlighted there is the fact that all dehydrogenation reactions are easier with light and early transition metals. On the other hand, hydrogen removal from the different positions in the carbon chain is not

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In more complex reactions, the catalytic site may require a combination of surface species in close proximity on the surface. This certainly appears to be the case for the partial oxidation of hydrocarbons.116 The balance between dehydrogenation and dehydration reactions in organic alcohols provides a classic example of the central role that chemical kinetics plays in controlling selectivity in catalysis: while acidic oxides such as g-alumina often promote alcohol dehydration, basic oxides such as magnesia and calcium oxide typically favor alcohol dehydrogenation instead.117 A similar trend is often seen with alkanes, in particular when those reactants form the same initial surface alkoxide intermediates as the alcohols.54,116 Kinetic control in these systems requires the design of catalysts with chemically optimized active sites. In our laboratory we have been investigating the mechanistic issues related to this hydrocarbon oxidation catalysis by using model systems, nickel oxide model surfaces in particular. Potential catalytic surfaces, with oxygen atoms in different coordination environments and with different electronic properties, have been prepared by oxidation of nickel substrates



under vacuum, and characterized by both physical and chemical means.118–120 Defective oxide sites have been emulated either by covering open surface structures with subsaturation layers of atomic oxygen, or by purposely damaging crystalline NiO films.121 Additional surface hydroxide species have been produced by appropriate treatments with water vapor.122,123 These and other approaches have provided a versatile set of tools for the preparation of surfaces with a variety of sites. The active sites on those model oxide surfaces have then been probed using a combination of surface-sensitive techniques. Chemical methods in particular have proven quite informative for the characterization of local surface environments. For instance, carbon monoxide was shown to be an excellent local probe for the determination of the oxygen coordination and oxidation state of individual nickel surface atoms.121 This is illustrated by the data in Figure 5, where a direct correlation is indicated between the oxidation state of particular surface nickel atoms and the energy of adsorption of CO on those sites.121 Analogously, ammonia was used as a chemical probe for catalytic surfaces, in that case to characterize the acidity and reactivity of hydrogen atoms around oxygen-containing sites, and also the ability of those sites to promote H¶D exchange reactions.124,125 The oxide sites prepared and characterized by the methodology described above have in many instances displayed unique

CO Adsorption Energy / kcal·mol -1

30

CO TPD Titration of Ni Sites on NiOx

25

20

(3x1)O-2

15

NiO(100)

Ni(110)

reactivity toward hydrocarbon conversion processes. In the case of the oxidation of 2-propyl intermediates, for instance, acetone formation takes place on sites fulfilling a number of criteria,126–128 namely: (1) the availability of a nickel atom for the initial dissociative adsorption of the reactant neighboring labile oxygen atom capable of inserting into the nickel–alkyl bond to form an alkoxide surface intermediate;129 (3) the ability to preferentially promote a b-hydride elimination step from that alkoxide at higher temperatures to yield either an aldehyde or a ketone; and (4) the presence of surface hydroxide groups nearby to further promote this partial oxidation pathway. Similar conversions have been seen with other alkenes and carbenes and on other crystallographic planes,119,130,131 and also when starting from alcohols (which can be easily dehydrogenated to the same alkoxide intermediate). This suggests that the above observations may be quite general, and may provide a fairly detailed if tentative picture of the nature of the active site required for selective partial oxidation catalysis. Particularly active sites can be generated by using low oxygen doses on (110) surface planes of late transition metals. A number of past studies have indicated that this treatment leads to the formation of incomplete and quite mobile {¶metal¶O}n surface rows with highly unsaturated terminal oxygen atoms.132 Examples of the unique reactivity of those sites are given by the appearance of a new strong binding state for ammonia124 and by their ability to promote the hydrogenation of surface methyl groups to methane,131 as shown in Figure 6. Using xenon as a probe for the electronic properties of local surface ensembles,133–135 we have been able to explain the high activity of those sites by their unique local electrostatic potential. This is highlighted by the high temperature of Xe desorption seen in the TPD data in the insets in Figure 6.136 When combining the results from the titration, chemical, and physical probing procedures with reactivity studies, it becomes quite clear that the electronic and structural properties of specific oxide sites define their selectivity toward individual reactions in catalysis.

(3x1)O

10

(2x1)O

Surface Changes Under Catalytic Conditions

5 0.0

0.5 1.0 1.5 Estimated Ni Oxidation State

2.0

Fig. 5. Correlation between carbon monoxide adsorption energy, as obtained from temperature-programmed desorption (TPD) data, and nickel oxidation state for the case of Ni(110) single-crystal surfaces oxidized with increasing amounts of O2.121 The five ordered structures that form in this system are also shown for reference. A clear and significant drop in adsorption energy is seen as the surface becomes more oxidized, an observation that makes CO an ideal chemical probe for local oxidation states. Given that the behavior reported in this figure can be reversed by ion sputtering of the final NiO crystalline films, it can be concluded that the O¶Ni(110) surfaces are reasonable models for more realistic defective nickel oxide catalysts.


So far, a number of structural and electronic issues have been identified in this review as central for the definition of the nature of surface sites, and with that the selectivity of catalytic processes. In addition, the properties of the surface can be affected further by changes occurring under catalytic conditions, typically atmospheric pressures and/or high temperatures. It is known that the adsorption of reactants may cause restructuring of the surface,137 and in alloys and other complex solids the preferential segregation of some of their components.138 The extent to which those changes affect activity and

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NH3 and CH4 TPD From O/Ni(110)

Yield / arb. units

Role of Terminal Oxygens in –Ni–O Rows

CH4 from CH3 I Xe TPD 80K

3.0L O2

Xe TPD

NH3 Tdes=400K

0 0.01

100K

70

0.1

70

T/K

150

0.1L O 2

T/K

150

1 O 2 Exposure / L

10

Fig. 6. Temperature-programmed desorption (TPD) yields as a function of oxygen predose for high-adsorption-energy (400 K) molecular ammonia124 and for methane production from surface methyl moieties131 on Ni(110) singlecrystal surfaces. The oxygen exposures were carried out above 300 K to produce atomic oxygen layers with the added-row structures shown in Figure 5.132 For both reactions, peak activity is seen on surfaces with ~0.2 L of O2, which corresponds to the formation of incomplete, highly mobile, and oxygen-terminated {¶Ni¶O}n rows. The unique electronic properties of the terminal oxygens in these structures, the most likely sites for the chemistry reported here, is highlighted by the high desorption temperature seen for coadsorbed xenon atoms in the TPD traces displayed in the insets.136 These sites are good models for the defective oxides needed for the catalytic selective oxidation of hydrocarbons.

selectivity depends on the nature of the reaction.57 At one end, demanding reactions such as ammonia synthesis and CO, NOx, and hydrocarbon oxidations are typically operated under extreme pressures and temperatures, and most likely involve catalytic surfaces covered with small concentrations of strongly bonded species. The intermediates associated with such reactions are believed to be the same as those isolated and characterized by vacuum surface analytical techniques. Therefore, the chemical knowledge acquired from surface-science studies can in most cases be transferred directly to catalysis. At the other extreme, non-demanding reactions, hydrogenation of unsaturated hydrocarbons in particular, take place under milder conditions (atmospheric pressures, room temperature), and are promoted by surfaces passivated by strongly held spectator species. Those present a much bigger challenge for surface scientists, because the structural and electronic information obtained by studies under vacuum may not translate directly into what is observed during catalysis. Our research has clearly corroborated the past suggestion that the surface of the working catalyst in most hydrocarbon conversion processes is not pristine, but rather covered with

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a complex layer of strongly adsorbed hydrocarbon residues.59,60,62,139–142 This carbonaceous surface layer develops rapidly upon thermal decomposition of the reactants, and in the case of olefins consists of alkylidyne species74,143–147 produced via the dehydrogenation of alkylidene intermediates.110,148–150 Some experimental evidence suggests that such alkylidynes may be directly involved in demanding hydrogenolysis and reforming reactions,85,151 but for the milder catalytic hydrogenation of unsaturated hydrocarbons they are likely to play only an indirect role.60,61,142 Temperature programmed desorption and 14C radioisotope labeling experiments have indicated that the carbonaceous deposits that form during catalytic reforming become more hydrogen deficient and are held more strongly on the surface with increasing reaction temperatures.152 Nevertheless, they always turn over at rates orders of magnitude slower than those of most catalytic hydrogenation processes.74,143,153 Consequently, their most important contributions to the catalytic cycle are likely to be their ability to temper the high activity of the clean metal and to modify the chemisorption characteristics of the reactants. This is particularly clear with unsaturated hydrocarbons, for which a new p bonding state becomes available.154–156 Molecular beam data from our laboratory have allowed us to unravel some of the kinetic issues associated with these weakly adsorbed species. As an example of the experiments and analysis involved in those studies, in Figure 7 we show the time evolution of both the rate of adsorption of normal ethylene (C2H4) on a hydrogen-covered Pt(111) surface and the kinetics of C2H4 adsorption and perdeuteroethylene (C2D4) desorption after switching to a C2D4 beam, all at 230 K.157 The initial adsorption occurs with a high sticking probability, and produces a di-s strongly bonded ethylene layer. Once that state is saturated, however, a second weaker and reversible p species develops. The data in the bottom panel of Figure 7 prove that most of the adsorbed C2H4 can be displaced by new incoming C2D4, but at a much slower rate than that seen in the initial uptake. More importantly, a repopulation of the reversibly adsorbed ethylene is seen in the first 2– 3 s after the isotope switching, before the beginning of the desorption of the displaced C2H4. Displacement of adsorbates by other gases has been seen in the past,21,158 but in this case an interconversion between two different adsorption states is required. Further studies have revealed that the p-bonded species are in fact the ones directly involved in catalytic hydrogenation processes.86 The preceding observations were used to build a model for the hydrocarbon reforming catalyst.59–62,139 As stated above, the working surface becomes covered with a strongly held hydrocarbon layer within a short time after the beginning of the catalytic process. This layer often displays a complex structure that includes both regions with three-dimensional growth and uncovered bare metal patches,159 and may even block



0.04

Ethylene/Pt(111)

0.03

Adsorption and Exchange Kinetics

0.02

A. Initial C2H4 Adsorption

0.01

T = 230 K H Precoverage = 0.5 ML Flux C2 X4 = 0.04 ML/s

-1

Rate / ML·s

isomerization/cyclization products, respectively. Those processes are likely to occur on uncovered metal sites, and to follow the same chemistry seen under vacuum.57 The implicit balance needed between mild hydrogenation–dehydrogenation reactions and more demanding isomerization and cyclization steps suggests that the performance of hydrocarbon reforming catalysts may be improved by fine-tuning the buildup of the carbonaceous deposits. This is typically achieved by the use of metal alloys141,162 and/or by the addition of modifiers such as sulfur, alkaline metals, and chlorine.163,164 Here is another example of how the reaction mechanism defines the requirements for the catalytic site.

0.00 C2D4 adsorption

0.01

B. C2H4 Displacement by C2D4

0.00 Net rate change

-0.01

C2 H4 desorption

-10

0

10

20

30

40

50

Time / s Fig. 7. Top: temporal evolution of the rate of ethylene adsorption on a hydrogen-predosed Pt(111) surface at 230 K.86,157 The high and approximately constant sticking coefficient seen for most of the uptake corresponds to the buildup of a di-s bonded layer. Bottom: rates for both C2H4 desorption and C2D4 adsorption during exposure of a C2H4-saturated surface to a C2D4 beam. Significant exchange of adsorbates is possible in this system, but at a much slower rate than the initial uptake. Moreover, a net increase in surface coverage is seen within the first few seconds of exposure to the C2D4 beam, as highlighted by the spike seen in the trace for the overall rate change. This adsorption is associated with the population of a weakly p-bonded ethylene state, which additional experiments indicate is the species directly involved in the hydrogenation to ethane. The original di-s ethylene is a spectator species whose main role is to passivate the activity of the metal to allow for the formation of weaker adsorption states.

surface defects preferentially, rendering non-demanding reaction essentially structure insensitive.159 During catalysis, the incoming reactants are initially activated on bare metal sites and converted into surface intermediates, into adsorbed alkyl species in the case of alkane activation.64,160,161 The majority of those alkyl surface species then undergo rapid and reversible b-hydride elimination, and produce the alkenes seen in the fast alkane–alkene equilibria that accompany most reforming processes. The mild and rapid nature of the bdehydrogenation steps suggests that they take place on surface areas modified by the carbonaceous deposits. On occasion, however, some alkyl fragments may undergo more demanding a- or g-hydride eliminations, to form alkylidene or metallacycle intermediates, and ultimately to yield hydrogenolysis and


Effect of Catalytic Conditions Clearly, carbonaceous deposits modify the behavior of hydrocarbon conversion catalysts, but this modification can be seen as unintentional, since the buildup of those residues occurs only because of the decomposition of the reactants on the clean metal surface at the start of the catalytic process. However, catalysts can also be modified intentionally. There are indeed many instances of industrial processes that rely on the use of additives such as alkali metals, halogens, or inorganic oxides to either affect the electronic properties of the catalyst or stabilize a particularly active solid phase. A more subtle approach is the use of molecular modifiers to imprint specific properties to the active catalytic surface. To further discuss this approach, it is worth returning to the example involving the addition of small amounts of chiral cinchona to the reaction mixture to make the hydrogenation of a-keto esters to their corresponding a-hydro ester over supported platinum catalysts enantioselective. This modification has been explained by the formation of a complex between the quinuclidine moiety of the cinchona and the reactant to force the latter into a particular adsorption geometry on the surface with only one side of the carbonyl plane available for hydrogenation.165,166 Based on in-situ infrared spectroscopy studies at the liquid–solid interface,46,167 where this chemistry takes place, we have reached the conclusion that the performance of the cinchona-modified platinum system is mainly defined by the characteristics of the adsorption of the modifier itself on the metal, and that those can be controlled by the conditions used in the catalytic process.44–47,168,169 For one, optimal enantioselectivity appears to correlate with the concentration of the modifier, as discussed above (Figure 2).44,45 Also, the catalyst needs to be pre-treated with hydrogen before the adsorption of the cinchona and the catalytic reaction can start.168 Lastly, both the cinchona adsorption and the activity and enantioselectivity of the cinchona/platinum system correlate well with the polar character of the solvent and with the solubility of the chiral modifier in it (Figure 8).169 In general terms, the use of

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Cinchonidine/Pt Ease of Desorption vs. Solubility CH2 Cl2

mL / V1/2

0.1

Chlorobenzene

Pt Surface

0.01 Ethyl Ether

optimizing activity and/or selectivity are the size and geometrical arrangement of the cluster of surface atoms required for the promotion of the desired reactions. Electronic properties are to be given similar considerations when designing catalysts. Finally, the performance of the catalyst can in some cases be defined by controlling the reaction conditions, in particular when the nature of the surface sites may be modified under high pressures or extreme temperatures. Funding for the preparation of this manuscript has been provided by grants from the US National Science Foundation and the US Department of Energy.

CCl4

REFERENCES Solvents

0.001 Cyclohexane

0.1

1 -1 Solubility / g·L

10

Fig. 8. Correlation between the solubility of cinchonidine in different solvents and the strength of its adsorption on a platinum surface.169 The latter was determined by reflection–absorption infrared spectroscopy (RAIRS) experiments, by measuring the volume of the pure solvents required to dissolve and remove a saturated cinchona monolayer from the surface. The data indicate that the solvents that best dissolve cinchonidine are also the ones that remove the monolayer faster. The differences in the extent of reversibility of this adsorption seen with the various solvents nicely explain similar changes reported for the catalytic conversion of a-keto esters using the cinchona/platinum system. This is an example of how the reaction conditions can be used to tune and optimize the catalytic sites.

complex chemical compounds as modifiers can be viewed as a way to create specific catalytic sites by “pre-fabricating” them using discrete molecular units.

Concluding Remarks To conclude, it is worth reiterating the main thesis of this report, that selectivity in catalysis may be controlled by subtle tuning of the catalytic site, and that the requirements for such tuning are defined by the mechanistic details of the reactions involved. Examples were provided above for cases where the active surface ensembles demand complex stoichiometries or chemical compositions. The required catalytic sites can be built in situ on the catalytic surface, or provided by external modifiers such as molecules with desirable structural features. Instances were also mentioned in which the main criteria for

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